Friday, September 26, 2014

The Egg Masses of Freshwater Pulmonate Snails

Faithful readers of this blog will not be surprised to
learn that my email inbox typically receives a rather steady stream of
inquiries with attached jpeg images of freshwater snails. But you might
be surprised to discover that I also occasionally receive images of things that
are not freshwater snails, but could be.

Like blobs of jelly. Earlier this year, for example, I received an
email from Mr. Tom Pelletier of askanaturalist.com, bearing the subject line
“gelatinous mass.” And the attached
image was (pretty clearly) a Physa egg mass, sent to Mr. Pelletier by a
correspondent who had photographed the underside of a river rock in West
Virginia. I was (of course) pleased to
help Mr. Pellatier, and he wrote a very nice essay on the askanaturalist.com
website [1], featuring a lot of excellent general information on the life
history of Physa acuta, as well as photos and a couple video links as well.

So it has come to my
attention that reliable information on the egg masses of freshwater pulmonate
snails is a rare commodity on the web. I
tried a google search on a variety of terms and combinations, and was only able
to find our colleague Kathryn Perez’ dichotomous key (which is good), but which
features drawings, rather than photos, and might benefit by attention to scale
[2].

So welcome to
“Basommatophoran Pulmonate Egg Masses 101.”
This is an introductory class. If
you are a sophomore or higher in the freshwater gastropod curriculum, feel free
to take the rest of this essay off, and I’ll see you next month.

For those of you still
with me. Last week I went down to my
local pond and collected adults from the three most common pulmonate populations
in the Charleston area – Lymnaea (Pseudosuccinea) columella, Physa acuta, and Helisoma trivolvis. I isolated individual snails
in my standard 10 oz. plastic drinking cups, fed them green flake food, and
over the following 48 hours, quite a few laid eggs. I allowed the eggs to mature for five days, dumped
the water from their cups, and trimmed representative egg masses out with a
pair of scissors, still attached to their cup walls. I then photographed one image directly down
through each mass, and a second image obliquely (dewatered, propped up in a little
finger bowl) to give a feeling for third-dimension thickness.

The photo above shows a
typical lymnaeid egg mass about five days old.
The clear, colorless, elongate, sausage-shaped mass depicted is
approximately 12 mm in length, each of the 25 - 30 embryos it contains
approximately 0.7 mm in diameter. It is
covered with a relatively tough membrane.
The standard shell length of its mother was 11.5 mm.

The second photo in this
series shows a typical planorbid egg mass, also about five days old. The 25 - 30 embryos are very similar in size
to the lymnaeid embryos depicted in the first photo, but the mass is irregularly
ovoid in its outline, with a maximum dimension of about 7 mm. This mass is also covered with a relatively
tough membrane. Its mother bore a shell
13.2 mm in diameter.

Notice, interestingly,
that the planorbid egg mass is tinged slightly brown or orange, in contrast
with the entirely uncolored lymnaeid mass.
Planorbids are famous for their serum hemoglobin, and it seems likely to
me that the slightly orange cast may indicate a bit of heme in the matrix. The development of Helisoma embryos also
seems a bit more advanced at day five than lymnaeid embryos.

The third photo in this
series depicts a typical physid egg mass, similar in size and age to the
lymnaeid and planorbid masses. The
matrix in the Physa egg mass is obviously much more gelatinous in its
character, however, missing the relatively tough outer membrane. The standard shell length of the mother of
this brood was 9.1 mm.

To be complete, I might add
a fourth image to the gallery, depicting a singleton egg of the ancylid limpet
Ferrissia fragilis[3]. Ferrissia
reaches maturity at a much smaller size than Lymnaea, Helisoma, or Physa, and
hence one might not be surprised to discover they do not lay an egg “mass” at
all. The singleton embryos of Ferrissia are
a bit smaller than the embryos of the other pulmonates as well, approximately
0.60 mm diameter, and surrounded by a rather spare capsule.

I will conclude this
lesson by noting that the size of freshwater pulmonate egg masses is a function
of their number of embryos, which may vary greatly. In culture it is not uncommon to see Physa
egg masses with 60-80 embryos, for example, roughly comparable in total volume
to that of their mother. And production
of one such egg mass every 24 hours is not unusual.

Even casual observations
such as these cannot fail but impress the student with the potential for great
reproductive output mounted by freshwater pulmonate snails. Might pulmonates “over-reproduce” and expire,
like spent salmon? Readers interested in
a comprehensive review of life history strategy in freshwater gastropods
generally, together with a consideration of spent-salmon semelparity, are
referred to my (2000) book [4].

Thus ends the introductory
lecture on the egg masses of freshwater pulmonates. Coming up next month – advanced topics!

5 comments:

Dear Rob, thanks for still including me in your FWGNA list. You may have not heard from me in a while (sorry about that!), but I always follow your blog. Very interesting blog about pulmonate eggs! I have recently dedicated some research effort into snail ecology at egg stage. The first output has appeared in FAL (Fundamental and Applied Limnology) in 2011; more stuff to follow soon. The FAL paper includes egg clutch pix — anybody interested is welcome to visit the FAL webpage (subscription required for PDF download), or please e-mail me for a reprint at physa@tiscali.it or p.lombardo@limnoconsulting.com.

I was using Helisoma trivolvis and Lymnaea columella. And you are correct, the Helisoma individual was on the small side of adulthood for the Charleston area this time of year. But the Lymnaea individual was actually on the large side. This odd distribution of adult sizes resulted from my efforts to illustrate egg masses with matching numbers of embryos.